In optical telecommunication systems, one of the many difficulties encountered is the chromatic dispersion of light signals propagating over long distances in optical fibers. The chromatic dispersion in non-dispersion-shifted optical fiber is nominally 17 ps/nm/km in the 1550 nm telecommunication window, but this value changes as a function of the wavelength: its value changes by about 2 ps/nm/km between 1530 nm and 1565 nm. Several single-channel dispersion compensators based on Fiber Bragg gratings (FBGs) have been proposed. Although they were demonstrated to be appropriate solutions for compensating the chromatic dispersion in a single WDM channel, in multi-channel systems, the spectral variation of the chromatic dispersion must be taken into account, especially for data transmission systems operating at high rates such as 10 and 40 Gbit/s. There is therefore a need for a broadband dispersion compensator that compensates for the chromatic dispersion but also for its spectral variation. This feature is often referred to as the dispersion slope compensation.
Fiber Bragg gratings are a well established technology for the fabrication of components for optical telecommunications, especially for WDM. Basically, a Bragg grating allows light propagating into an optical fiber to be reflected back when its wavelength corresponds to the grating's Bragg wavelength, related to its period. A chirped Fiber Bragg Grating, in which the Bragg wavelength varies as a function of the position along the fiber, represents a well known solution for compensating the chromatic dispersion of an optical fiber link (F. Ouellette, <<Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides,>> Opt. Lett., 12, pp. 847-849, 1987; R. Kashyap, <<Fiber Bragg gratings,>> Academic Press, 458p., 1999). This spatially variable Bragg wavelength is defined as the product of twice the spatially variable grating period and the fiber effective index, which can also be spatially variable. Such a chirped grating compensates for the accumulated dispersion by reflecting light with a group delay that varies as a function of the wavelength. An appropriate grating can be fabricated such that the wavelength dependence of its group delay is just the opposite of that of the fiber link. Different solutions based on FBGs have been proposed for broadband dispersion compensation but most of them do not include the slope compensation.
Referring to M Durkin et al. <<1 m long continuously written fibre Bragg grating for combined second- and third-order dispersion compensation>>, Electron. Lett. 33, pp 1891-1893 (1997) and J. F. Brennan et al. in BGPP 1999, pp.35-37, ultra-long FBGs, up to 10 m long, have been demonstrated for dispersion compensation over a large bandwidth. However, such devices suffer from high group delay ripples and do not allow an adjustment of the dispersion. The group delay of a compensator based on ultra-long FBGs is schematically illustrated in FIG. 1 (prior art). The chromatic dispersion the device compensates for is given by the slope of the group delay. The example shown in FIG. 1 has a dispersion of —1250 ps/nm and thus compensates for the chromatic dispersion accumulated over a 73 km long fiber link.
Sampled FBGs and Moiré FBGs have also been proposed in U.S. Pat. No. 5,384,884 (KASHYAP et al.) In particular for multi-channel dispersion compensation (see for example A. E. Willner, et al., <<Tunable compensation of channel degrading effects using nonlinearly chirped passive fiber Bragg gratings,>> IEEE J. of Selected Topics in Quantum Electron., 5, pp.1298-1311 (1999); U.S. Pat. No. 5,982,963 (FENG et al.); A. V. Buryak et al., <<Novel multi-channel grating designs>>, Proceedings of BGPP 2001; and M. Ibsen et al., <<Chirped moiré fiber gratings operating on two-wavelength channels for use as dual-channel dispersion compensators,>> IEEE Photon. Technol. Lett., 10, pp.84-86, (1998)) in which the sampling function replicates a given dispersion function (M. Ibsen et al, <<Sinc-sampled fiber Bragg gratings for identical multiple wavelength operation>> IEEE Photon. Technol. Lett., 10, pp.842-844, 1998). As a result, all the channels are identical and the device cannot compensate for the dispersion slope. The group delay of such a compensator based on a sampled FBG is schematically illustrated in FIG. 2 (prior art). An approach for multi-channel slope compensation has been proposed based on interleaved sampled Bragg gratings in W. H. Loh et al. <<Sampled fiber grating based dispersion slope compensator>>, Photonics Technol. Lett. 11, no 10, pp 1280-1282 (1999). This theoretical approach is expected to suffer from significant practical difficulties associated with the control of the many micro-grating structures.
Single-channel non-linearly chirped FBGs have been proposed for narrowband dispersion slope compensation (J. A. R. Williams et al., <<Fiber Bragg grating fabrication for dispersion slope compensation>>, IEEE Photon. Technol. Lett., 8, pp. 1187-1189, 1996). In order to achieve operation over a broader range, multi-channel non-linearly chirped FBGs were proposed (Y. Xie et al., <<Tunable compensation of the dispersion slope mismatch in dispersion-managed systems using a sampled nonlinearly chirped FBG>>, IEEE Photon. Technol. Lett., 12, pp.1417-1419, 2000). In this approach, the replicated bands of a sampled FBG have a different wavelength separation than the WDM channels, causing each channel to experience a different dispersion compensation.
In view of the above, there is a need for a dispersion compensating device that takes into account the slope of the dispersion experienced by a broadband multi-channel signal.
In addition, critical factors such as changing traffic patterns, temperature fluctuations along the fiber, component dispersion levels and dispersion variations in the transmission fiber (from manufacturing variances) affect dispersion compensation, especially at high bit rate. To accommodate these factors, 40 Gb/s systems and some 10 Gb/s systems require not only fixed, broadband slope-compensated dispersion-compensating devices, but also tunable dispersion technology to adjust the dispersion compensation in real-time for different WDM channels.
The wavelength of peak reflection for a Bragg grating can be shifted by a change in either the strain or the temperature (or both) imposed on the grating. If the grating is subjected to a strain or temperature gradient, the modulation period of the index of refraction and the mean index of refraction become functions of position along the grating.
If a linearly chirped FBG is uniformly stretched, its period is changed, and accordingly the Bragg reflection wavelength is also changed, but the dispersion remains unchanged. A similar situation pertains if, instead of stretching the fiber, a uniform heating is applied to the grating. On the other hand, a non uniform heating, such as to produce a thermal gradient along the waveguide axis in the region of the grating, induces a chirp in the grating. Controlling the magnitude of the thermal gradient controls the magnitude of the resulting chirp, and thus there is provided a form of adjustable dispersion compensation device. Such a device is for instance described by different implementations described hereafter.
U.S. Pat. No. 5,671,307 (LAUZON et al.) discloses the use of a temperature gradient to impose a chirp on a FBG. By inducing a uniform linear variation of the local temperature over the length of the FBG, a slope variation of the time delay can be obtained, resulting in a variation of the dispersion compensation. The temperature gradient is realized by providing heat conductive means such as a thin brass plate to hold the portion of the fiber provided with the Bragg grating, and pairs of Peltier effect plates sandwiching each end of the fiber to selectively apply and dissipate heat to and from the ends of the fiber. Lauzon suggests that the device might be used as an accurately tunable dispersion compensator for optical fiber communication links. However, the proposed device lacks the power efficiency required to make it practical. This purely thermal approach has the advantage of avoiding any stresses in the fiber.
Based on the same idea, European patent No. 0 997 764 (EGGLETON et al.) discloses an optical waveguide grating with adjustable chirp formed by a waveguide grating in thermal contact with an electrically controllable heat-transducing body which varies the temperature along the length of the grating. The heat transducing body, formed by example by a tapered film coating whose resistance varies along the length of the grating, can generate heat on the fiber to establish a temperature gradient along the grating.
It is also known in the prior art to use a plurality of localized heaters along the length of a chirped FBG to alter its properties in order to tune the chirp and to produce tunable dispersion compensators. U.S. patent application Ser. No. 2002/048430 (HASHIMOTO) presents such an approach where an optical fiber is coupled to a succession of localized heaters mounted on a substrate. Linear temperature profiles are obtained which tune the dispersion in the linearly chirped FBG placed in close contact. The plurality of localized heaters can also be controlled such as to produce a non-linear temperature profile along the grating.
The previous embodiments refer to the application of a thermal gradient to a fiber Bragg grading. Similarly, if the waveguide is subjected to a stretching that is not uniform, but is such as to produce a strain gradient along the waveguide axis, then the effect is to produce a controllable amplitude of chirp. Imai (T. Imai et al., <<Dispersion Tuning of a Linearly Chirped Fiber Bragg Grating Without a Center Wavelength Shift by Applying a Strain Gradient>>, IEEE June 1998, pp. 845-847) and U.S. Pat. No. 6,360,042 (LONG) describe devices in which a strain gradient is imparted to an optical fiber waveguide by bonding a portion of its length to a cantilever, and then bending that cantilever. U.S. Pat. No. 5,694,501 (ALAVIE) is another example of such a device in which a strain gradient is imparted to an optical fiber by cantilever bending and also by bonding it to the side of a stack of electrostrictive elements, and then applying a differential drive to those elements. The use of magnetostriction for grating chirping can also be used, as disclosed by U.S. Pat. No. 6,122,421 (ADAMS et al.). This patent discloses a programmable and latchable device for chromatic dispersion compensation based on a gradient magnetostrictive body bonded along the length of the fiber grating. In such a device, the magnetic field causes the body to expand or contract depending on the material. Alternatively, European patent no. 0 867 736 (FARRIES et al.) discloses a temperature-based device that combines the application of a temperature gradient and a strain to modify the optical properties of the grating. All of these devices however imply gluing the fiber to a metallic block along its entire length, which in practice is a technologically challenging operation.
The uniform stretching of an optical waveguide possessing a chirped Bragg grating with a quadratic component of its chirp can also induce a change in the linear dispersion afforded by the structure, as described in U.S. Pat. No. 5,982,963 (FENG) and Y. Xie et al., <<Tunable compensation of the dispersion slope mismatch in dispersion-managed systems using a sampled nonlinearly chirped FBG>>, IEEE Photon. Technol. Lett., 12, pp.1417-1419, 2000. This approach allows a tuning of the dispersion but the spectral duty factor is limited to about 25%. Furthermore, this method relies on mechanical stretching which may cause fiber fatigue and degrade long-term reliability.
Another tunable dispersion compensator based on uniformly straining quadratically chirped FBGs is presented in U.S. Pat. No. 6,363,187 (FELLS) and in U.S. Pat. No. 6,381,388 (EPWORTH). In an effort to combat the transmission penalty associated with a quadratic chirp, this patent uses the reflection in a second Bragg grating identical to the first, but oriented to provide a quadratic component of chirp that has the opposite sign to that of the first Bragg reflection grating, and with a substantially matched modulus.
All of the above prior art techniques have their advantages and drawbacks, but none provides a simple optical structure which allows for the tunable dispersion and dispersion slope compensation of a multi-channel light signal.